JPH0671106B2 - Laser beam monitor for laser system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a laser beam monitoring device in a laser system, preferably a large-scale laser system, and more particularly to a laser beam capable of detecting a laser beam position and a laser mode. The present invention relates to a monitor device.

[Prior Art] A typical large-scale laser system is a glass laser system for nuclear fusion research. An example of the configuration of such a laser system is shown in FIG. 2 (The Institute of Electrical Engineers of Japan, C, 107, 5
No., 1987, pp. 455-462). Laser light from the laser oscillator OSC is guided to the target chamber through the preamplifier array X array, the main amplifier array Y array, and the Z array. In this process, the laser light passes through an optical shutter, a Faraday rotator, a spatial filter, etc., and the laser intensity is increased by a plurality of amplifiers while being mode-formed. Therefore, the propagation distance from the laser oscillation stage to the target chamber is 270 m,
Deviation angle of beam optical axis from ideal optical axis (beam pointing error angle)
However, strict optical axis adjustment of less than 20 μrad is required. Therefore, in the above laser system, in order to save the labor and improve the reliability of the laser optical path adjustment, the four-division type photodetector is used, and the automatic optical axis adjustment is performed by the return control by the microcomputer. The laser optical axis adjustment is performed using a four-division type photodetector (denoted as a centering detector in FIG. 1) shown in FIG. 3 (a) and a pointing detector shown in FIG. 3 (b). ing. As shown in FIG. 1, these are arranged so as to receive the excessive light of the laser light reflecting mirror, and the four-division type photodetector divides the equilibrium state of the laser light intensity of the photoelectric surface divided into four quadrants into The beam position is detected. The pointing detector focuses the laser light on a position detector (PSD) with a lens and monitors the beam optical axis deviation angle from the output of two pairs of current terminals. The beam position determination by the former four-division type photodetector is performed by adding and subtracting the signals of the four photocathodes on the assumption that the laser light intensity distribution is axially symmetric (TEMoo mode).

[Problems to be solved by the invention]

However, the above-mentioned conventional technique cannot accurately detect the intensity distribution (laser mode) in the cross section of the laser beam, particularly the TEMoo mode which is an important laser mode used in a large-scale laser system. It is difficult to deal with changes in the intensity distribution (laser mode) in the beam cross-section due to damage to the optical element, and it is not possible to adjust the laser optical path completely automatically because human confirmation work is required after the optical path adjustment is completed. There was a problem. Therefore, by providing the laser position detector with the laser mode measurement capability, the laser optical path can be adjusted while always checking the presence or absence of a failure in the optical element or laser amplifier in the laser optical path. It is desired to be able to adjust the laser optical path.

An object of the present invention is to provide a device capable of performing beam position detection while monitoring a laser beam mode by providing a laser beam position detector with a measurement capability of a radial laser intensity distribution in a laser beam cross section. It is in.

[Means for solving problems]

The above-mentioned object is to arrange a laser beam in a laser system in a coaxial ring split type laser beam detector that is composed of a photodetection unit at the center and a photodetection unit that is arranged in a ring shape around the photodetection unit and is circumferentially divided. This is achieved by making the incident distributed.

[Action]

The laser beam position can be measured by comparing the outputs of the ring-shaped split photodetectors of the coaxial ring split type laser beam detector. Further, by comparing the output of the photodetector at the center of the coaxial ring split type laser beam detector and the output of the ring-shaped split photodetector, the radial intensity distribution in the cross section of the laser beam is measured,
Whether or not the laser mode is TEMoo can be determined.

〔Example〕

FIG. 4 is an exemplary view of a coaxial ring split type laser beam position detector according to the present invention. This coaxial ring split type laser beam detector (hereinafter abbreviated as CR type PD) 1 has a circular detection unit at the center, and a ring-shaped detection unit is arranged around it. If the number of rings is m, then m ≧ 1.
The ring is divided into n in the circumferential direction (n ≧ 3), and FIG. 4 shows an example of n = 4.

Such a CR type PD1 is installed at a required position in the laser optical path to detect the beam mode (laser mode) and the beam position. To apply laser light to the CR type PD, use reflected light or transmitted light from existing optical elements in the laser system,
This should avoid unnecessary attenuation of the laser light and increase instability of the beam optical path. As an example, FIG. 5 shows an arrangement example in which the reflected light from the laser amplifier 7 and the transmitted light from the laser light reflecting mirror are utilized and are made incident on the CR type PD1. CR type PD1 has each amplifier 7 and mirror 5 as shown in the figure.
It is desirable to install each of them from the viewpoint of beam monitoring and control.

The time response of the CR PD1 needs to be sufficiently shorter than the required laser optical axis adjustment time. CR type with the shape shown in Fig. 4
PD1 can be easily manufactured by using a semiconductor photodetector, and high-speed time response characteristics of up to about 10 ⁻⁶ seconds can be obtained. This time response is the mirror adjustment time for laser optical axis adjustment ( ^10-10 seconds), which is sufficiently short. Therefore, the semiconductor CR PD meets the above-mentioned requirements. In addition, since the distance between the detectors in the semiconductor type CR PD can be reduced to about 10 ⁻⁶ m, the presence of the dead zone can be ignored when detecting the laser position.

As described below, the CR type PD1 can measure not only the position of the laser beam in a large-scale laser system, but also the radial and circumferential intensity distribution (laser mode) in the beam cross section. Since the laser mode monitoring in the laser system can be performed at the same time as the position detection, it can be detected when the cross-sectional intensity distribution of the beam changes due to breakage of a mirror or the like, and the automatic laser adjusting device does not malfunction.

Each example of a typical laser mode is shown in FIG. In FIG. 6, the TEMoo mode has the Gaussian laser intensity distribution shown in FIG. 7 and has good laser light propagation characteristics. The laser mode used in large-scale laser systems is required to be TEMoo. A conventional two-dimensional diode array laser detector divided into a checkerboard pattern is suitable for measuring modes other than TEMoo in Fig. 6, but this detector is complicated in the subsequent data processing. Also, it is not the optimum shape for detecting the most important TEMoo mode.

As described above according to the present invention, the CR type PD1 having an axially symmetric detector in the center and its periphery, divides the laser beam intensity distribution into several regions and outputs a signal proportional to the laser intensity integral value of each region. Output. The circular detector in the center of CR type PD1 plays an important role in confirming the TEMoo mode. That is, as shown in FIG. 6, in the laser modes other than TEMoo, the laser intensity at the center is smaller than that at the periphery. Therefore, whether or not the laser mode is TEMoo can be easily determined by monitoring the ratio of the laser intensity at the central circular detection unit and the laser intensity at the peripheral detection unit in the CR type PD1. Further, the beam position can be accurately detected by comparing the outputs of the ring-shaped detectors surrounding the central circular detector. TEMoo mode laser optical path adjustment is CR type P
The laser optical path mirror may be moved so that the outputs of the detectors arranged on the same circumference of D1 are all equal. Even if optical elements such as mirrors and amplifiers in the laser system are partially damaged and the laser intensity distribution partially deviates from the TEMoo mode, as will be described later, on the same circumference of CR type PD1. Erroneous adjustment of the laser optical path can be avoided by making the signals of the other detecting sections equal or by adjusting the laser optical paths so that the signals of the detecting sections on the other circumferences become the same.

Figure 8 shows the concept of the signal processing system from the CR PD1. The electric signal from each detection unit (the number m × n + 1) of the CR type PD1 is sampled and held 10, sequentially read by the multiplexer 11, A / D converted 12, and then transferred to the microcomputer 16. The computer 16 uses this signal to adjust the mirror 5 through the mirror drive system 4 so that the center of the laser light intensity coincides with the center of the CR type PD 1, and then obtains the radial and circumferential distributions of the laser beam intensity and RAM 15 It is compared with the ideal intensity distribution (TEMoo distribution) stored in. If the measured laser intensity distribution has a difference of more than the allowable value from the ideal intensity distribution, an error signal is transmitted to the main controller 17. The laser beam position and the timing of measuring the laser mode are determined by a sample trigger signal from the main control unit 17, and the optical path of the laser system is adjusted in order from the laser oscillation stage to the subsequent amplifier system.

Hereinafter, Examples 1, 2, and 3 in which the above-mentioned CR PD is applied to a laser system will be described.

Example 1 CR type having the simplest structure with a single ring-shaped detector
An embodiment of laser beam position and mode detection by PD will be described with reference to FIG.

The CR type PD1 in the present embodiment is composed of a circular photodetection section at the center and a single ring-shaped photodetection section divided into four. It is assumed that the intensity of the signal from the circular light detecting portion in the central portion is I ₀ and the intensity of the signal from the split ring detecting portion in the peripheral portion is I _{1, i} (i = 1 to 4). After adjusting the laser system first, the CR type PD1 is installed so that the reflected laser light from the amplifier surface and the transmitted laser light from the laser light reflector enter the center of the detector. (In FIG. 1, the reflected laser light 9 from the surface of the amplifier 7 is made incident on the CR type PD 1 through the lens 8.) The signal intensities I ₀ , I _{1, i} at this time are stored in the computer. This is used for beam mode (laser mode) measurement. As an example, the procedure for detecting the beam position incident on the amplifier 7 and mode detection will be described below.

The reflected laser light from the amplifier 7 is measured by the CR type PD1. Since I _{1, i} (i = 1 to 4) takes different values when the laser optical path changes, the illustrated mirror 5 is adjusted so that they become equal. At this time, even if the mirror (the illustrated mirror 5 or another mirror (not shown)) is damaged by the laser beam, the damaged portion is concentrated in the central portion where the laser intensity is large and does not affect the laser intensity in the peripheral portion. Therefore, the possibility of misadjusting the laser optical axis is small. Also, if the damage extends to the periphery, it
When the CR type PD1 stays within one detection area, the optical axis adjustment may be performed so that the other three detection section outputs I _{1, i} are equal, and the CR type PD1 is a flexible detector. Can be said.
If the damage extends to two or more detectors in CR type PD1, it can be diagnosed by the following laser mode measurement. The radius of the laser beam incident on the CR type PD1 is γ _l , the radius of the CR type PD1 is γ _D , and the radius of the center circular detection portion is γ _{D, 0} . Remember TEM ₀₀
Signal I ₀ from each detection unit of CR type PD1 when laser is incident,
I _{1, i} (i = 1 to 4) is compared with each signal during laser operation,
Monitor laser mode changes. γ _l = γ _D , γ
When _{D, 0 ~0.3γ D} becomes CR type PD1 TEM ₀₀ laser beam is incident, 0.4I after adjusting the beam position _{0 = I 1, i (i} = 1~4) comprising signal is detected. If the laser mode deviates from TEM ₀₀ due to damage to the laser oscillator or the mirror is damaged _, the ratio of I ₀ to I _{1, i} will change. You can

Embodiment 2 If the CR PD 1 described in Embodiment 1 is installed in each amplifier 7 as shown in FIG. 9, each amplifier gain measurement system can be constructed. For example, the amplification gain G ₁ of the # 1 amplifier 7 shown in the figure
Is More demanded. In this way, each amplifier gain G _i, which is important in a large-scale laser system, can be easily monitored using a CR type PD. In addition, the gain G _{i of} each amplifier (i = 1,2
It is also possible to automatically adjust the timing of the trigger to the amplifier so that () is maximized.

Example 3 The amplifier operating characteristic can be monitored by using the gain measurement system described in Example 2. In the coaxial pulse discharge excitation laser such as CO ₂ laser and metal vapor laser, the discharge parameter changes depending on the gas pressure in the laser tube and the discharge voltage. For example, when the gas pressure in the laser tube is high, the discharge starts in a part of the laser tube, and this tends to spread over the entire tube cross section with time. At this time, the time variation of the gain of the laser tube differs depending on the position of the laser tube cross section. When laser light is incident on the amplifier operated under such conditions, only the gain in the region corresponding to the discharge timing is high and the gain in other regions is low. The spatial distribution of the gain of this amplifier can be measured by using the CR type PD arranged as in the second embodiment. That is, in the second embodiment, the spatial average gain of the amplifier is measured from the sum of the outputs of the detection units of the CR type PDs, but in the third embodiment, the total of the outputs of the detection units of the CR type PDs is not calculated. Instead, the spatial distribution of the gain of the amplifier is measured from the comparison of the outputs of the individual detectors. This makes it possible to control the laser tube gas pressure so that the spatial distribution of gain becomes uniform.

By setting the number m of the rings of the CR type PD to 2 or more, the accuracy of laser beam mode measurement and laser beam position measurement can be improved. C in Examples 1, 2 and 3 above
The R-type PD1 has a simple structure in which the number of rings is 1 in order to use many R-type PD1s in the laser system, but on the other hand, it has a drawback that the accuracy of beam mode measurement and beam position measurement becomes rough. On the other hand, by increasing the number m of rings, the discriminating ability of the laser mode can be improved. When such a CR type PD is used, it is possible to adjust the laser optical path other than TEMoo. The radial intensity distribution in the laser beam cross section is measured by such a multi-ring PD, and a distribution curve that reproduces the intensity distribution is obtained by a calculator. After obtaining a region where the curve is a certain value close to the laser intensity 0,
The laser optical path is adjusted so that this matches the determined area of the amplifier and the mirror.

〔The invention's effect〕

According to the present invention, a laser beam mode, a laser beam position, and an amplifier gain in a large-scale laser system can be easily measured. is there.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a glass laser system for fusion research which is a typical example of a large-scale laser system, and FIGS. 3 (a) and 3 (b) are diagrams showing the same. 4 is a conceptual diagram of a conventional laser beam position detector used in FIG.
FIG. 6 is an exemplary view of a CR type PD according to the present invention, FIG. 5 is a view showing an arrangement example of the CR type PD, FIG. 6 is a view showing each example of a laser mode,
FIG. 7 is a diagram showing the radial laser intensity distribution of laser mode TEMoo, FIG. 8 is a schematic diagram of a CR type PD detection data processing system, and FIG. 9 is a schematic diagram showing laser amplifier gain measurement using a CR type PD. Is. 1 ... CR type PD, 2 ... signal memory 3 ... laser beam position / mode calculation computer 4 ... mirror drive system, 5 ... mirror 6 ... laser light, 7 ... laser amplifier 8 ... lens , 9 ... Reflected laser light.

Claims (3)

[Claims] 1. A photodetector provided in the center and a photodetector provided in a ring shape around the photodetector and divided in the circumferential direction (the number of the rings is 1 or more and the number of division in the circumferential direction is Three
In the laser system, there is provided a coaxial ring split type laser beam detector including the above) and means for making the laser beam in the laser system distributed and incident on the coaxial ring split type laser beam detector. Laser beam monitor device. 2. A laser beam monitor in a laser system according to claim 1, wherein the position and mode of the laser beam are monitored based on the output of the photodetector of the coaxial ring split type laser beam detector. apparatus. 3. The coaxial ring split type laser beam detector is provided on each of an incident side and an emitting side of a laser amplifier in a laser system, and a laser beam incident on the laser amplifier and a laser beam emitted from the laser amplifier are respectively provided. Distributing the light into the incident-side and exit-side coaxial ring split type laser beam detectors, and monitoring the spatial average gain or the spatial distribution of the gains of the laser amplifiers from the output of each coaxial ring split type laser beam detector. A laser beam monitor device in the laser system according to claim 1 or 2.